Biomolecules on solid surfaces have been investigated extensively to fabricate biosensors for a broad variety of applications. Amongst different strategies, affinity type biosensors are one of the most powerful and popular approach. Such sensors are based on the specific capabilities of a biorecognition element that is immobilized on a solid surface to selectively bind an analyte from a solution. They are versatile because they enable the determination of highly different species by selecting an appropriate biorecognition process, such as antibody-antigen interactions, complementary oligonucleotides interactions, and ligand-biological receptor interactions. One important aspect of such sensors is the minimization of non-specific binding to the sensor surface since such fouling increases the background noise, thus decreasing the signal to noise ratio and thereby the sensitivity. Detection of the signal from an analyte-biorecognition molecule complex is usually based on optical labels and probes of luminescence dye molecules. If the sensor is based on enzyme linked immunosorbent assay (ELISA) detection techniques, antibodies that are covalently linked to an enzyme such as horseradish peroxidase are used for detection. These antibodies either directly target the analyte or act as secondary antibodies to detect antibody-analyte complexes on the sensor surface. For detection of the signal, a substance is added that the enzyme can convert to a detectable signal like luminescence or fluorescence so that the amount of antigen in the sample can be determined.
Optical detection techniques, however, implicate the use of covalently labeled biomolecules if spatial resolution of the optical signal shall be accomplished. Such constructs are expensive and may restrict the assay due to difficulties in detecting certain biochemical activities. Moreover, the quantitative measurement of luminescence spectra sometimes requires complex and expensive instruments. Thus, alternative detection methods have been developed that exploit changes that either occur in the intrinsic physical property of the biomolecule itself or the interface between immobilized molecule and solid substrate as a result of its interaction with the target analyte. Such “label-free” biosensor assays are fairly straightforward, since an unlabeled biomolecule binds to an unlabeled analyte. Thus, numerous label-free biosensor systems have been developed with detection methods that make use of surface plasmon resonance (SPR), MALDI-TOF MS, electrochemical sensing, reflectometric interference spectroscopy, and quartz-crystal microbalances (QCM). Most of these label-free biosensors, however, require expensive and complicated equipments and are not straightforward to use. Moreover, sensitivity, robustness and the possibility to develop portable system are critical issues.